Cameras based on CMOS imaging arrays are rapidly displacing cameras based on CCD imaging arrays. The noise levels in CMOS cameras are central both to the cost of the camera and minimum light intensity at which the camera can operate. Each pixel includes a photodiode and a readout circuit. The readout circuitry has a finite noise floor that is dominated by 1/f noise. In addition, other sources of noise such as common mode noise due to transients in power supplies also contribute to the noise.
The signal from the photodiode is proportional to the amount of light that is intercepted by the photodiode during the image exposure period. This signal is proportional to the area occupied by the photodiode; hence, to overcome the noise from the readout circuitry at low light levels there is a tradeoff between increasing the size of the photodiodes, and hence, the imaging array and the minimum light intensity at which the array can operate. Since the cost of the imaging array is determined by the area of silicon in the CMOS imaging array, lowering the noise floor can provide either a decrease in cost of the imaging array or improved low-light sensitivity.
Many of the common mode noise sources can be reduced by providing more complex circuitry such as better regulated power supplies or power regulators on the imaging chips. However, these solutions also increase the cost of the imaging array. For low priced imaging systems such as those provided in cellular telephones and PDAs, these costs are significant.
In addition, the pixels exhibit significant dark current. That is, charge is accumulated even when the photodiodes are not exposed to light. This dark current presents problems at low light levels, and hence, makes it more difficult to design smaller pixels that can operate at low light levels. The contribution of the dark current to the pixel signals depends both on the exposure time of the image and the time between the closing of the camera shutter and the readout of the pixels. Hence, it must be measured separately for each exposure and readout. Again, the cost of correcting for the dark current increases the cost of the camera.